The present invention relates generally to a process for operating a plasma arc torch, and more particularly to a shut down process that extends the life of the electrode and nozzle.
The operation of conventional plasma arc torches is well understood by those in the art. The basic components of these torches are a body, an electrode mounted in the body, a nozzle defining an orifice for a plasma arc, a source of ionizable gas, and an electrical supply for producing an arc in the gas. As is well understood, upon start up an electrical current is supplied to the electrode (generally a cathode) and a pilot arc is initiated in the ionizable gas typically between the electrode and the nozzle, the nozzle defining an anode. Then, a conductive flow of the ionized gas is generated from the electrode to the work piece, wherein the work piece then defines the anode, and a plasma arc is thus generated from the electrode to the work piece. The ionizable gas can be non-reactive, such as nitrogen, or reactive, such as oxygen or air.
A common problem with conventional plasma arc torches is the significant wear of the electrodes and nozzles. Typically, the electrodes include a hafnium or zirconium insert. These materials are desired for their material properties when cutting with a reactive gas plasma, but are extremely costly and require frequent replacement. Typical plasma arc torches may incorporate cooling systems for the electrode, such as a cooling gas flow or water flow system for providing cooling fluids to the electrode. Although helpful, these cooling systems are not desirable in all applications of plasma arc torches and can limit the operational parameters of the torch.
The nozzle is another significant wear component. The nozzle is typically damaged by the plasma arc if the arc comes into contact with the nozzle once the plasma arc has been transferred to the work piece. In this regard, it is well known to induce a swirling component in the ionizable gas in the plasma gas chamber. This swirling gas flow stabilizes the plasma arc over its length from the electrode to the nozzle exit.
It has been found that a significant part of the electrode wear and nozzle damage actually occurs during shut down of the torch. It is believed that on cut off of electrical current to the electrode, wear results in a complicated interaction between molten surfaces of the electrode and the pressurized flow of the plasma gas through the nozzle. This phenomenon is also described in U.S. Pat. No. 5,070,227.
The '227 patent suggests a process for limiting the electrode wear upon shutdown of the torch wherein the mass flow rate of the plasma gas is reduced by closing off or reducing gas flow to the plasma chamber within a few hundred milliseconds before current is cut off to the electrode. The reduction in gas flow may be coupled with a venting of the plasma chamber to atmosphere to facilitate a more rapid change in the gas flow pattern in the plasma chamber. The '227 patent also teaches to alter the gas flow pattern by terminating the operational swirling flow and generating a generally radial flow at the input to the plasma chamber which becomes an axial flow through the chamber. This switch from the swirling flow to the radial/axial flow is in conjunction with the overall reduction in the flow rate and the decline in current level to the electrode. The '227 patent proposes that by substitution to a more axial flow, a higher overall mass flow rate can be tolerated at the time just preceding current cut off than with a swirling flow.
The present invention relates to a shutdown process for plasma arc torches that also minimizes electrode wear and damage to the nozzle.